Currently, in the United States the electrical power supply is most commonly provided using underground transformers. Typically, the transformers supplying power to an area are arranged into a loop system. One transformer loop system usually includes 8 to 50 transformers, and each transformer supplies power to 1 to 16 customers. Within the loop system, the transformers are sequentially connected one to another by a series of cables. Each cable is connected to the inlet of a transformer and the outlet of an immediate preceding transformer by two elbow terminators. The first and the last transformers are connected to a power source, such as an overhead power line. Within each loop system there is a normal open typically located at the middle of the loop system.
In operation, if a cable is fault, or a transformer has problem, a portion of or all transformers in the loop system will be out of service, and the customers will have power outage. The process of locating a fault cable or failed transformer is a time consuming and sometimes a very complex process. The time spent by the field investigators for locating the fault cable or failed transformer can be from 2 hours to 4 hours, depending on the size of the loop and the location of the fault. The lengthy power down time causes inconvenience to customers and financial loss to business.
To assist the diagnosis process, fault indicators have been used in the existing underground transformer loop system. The fault indicators are connected on to the cable right before the elbow terminator at the inlet of transformers. If a cable is fault, or the fuse in a transformer is blown by overload current, or fault current, the fault current travels back toward the power source. The fault indicators connected to the transformers preceding the failed transformer or the fault cable will sense the fault current and show an abnormal reading or displays a colored indicator. However, these fault indicators have been found not sensitive and their response is very unreliable. Furthermore, many existing loop systems do not have fault indicators installed, therefore, locating a fault cable or failed transformer frequently uses process elimination approach to gradually narrow down the possibilities.
To understand the difficulties associated with the existing diagnosis process, an example of locating a fault cable is provided. Assume an existing underground transformer loop system including 8 transformers (Tx1 to Tx8), each supplying 10 residential customers, therefore each transformer is more than two blocks away from the next transformer. The normal open is positioned at transformer Tx5. The problem is a fault cable between transformers Tx3 and Tx4. As the problem occurs, all customers supplied by transformers Tx1 to Tx5 are out of power supply, but the customers supplied by transformers Tx6 to Tx8 still have power as they locate on the other side of normal open.
As the customers call in to report power outage, an assigned trouble investigator needs first to verify that the lateral switch connected to the overhead power line before transformer Tx1 is open, which takes about 10 minutes because the lateral switch is commonly half a mile from the transformer loop. An open lateral switch means that the fuse in the lateral switch is blown by a fault current. The investigator reports to the dispatch his findings, and the dispatch check the loop system layout on the computer and verifies how many transformers within the loop system are out of service, which takes about 5 to 10 minutes, if no other accrued services are pending. The dispatch then instructs the trouble investigator to start working from the middle of the out of service portion using the process elimination approach. The investigator checks the fault indicators on transformers Tx1 to Tx5 if the transformers of this loop system have the fault indicators installed previously, otherwise, the investigator places fault indicators on each one of transformers Tx1 to Tx5. The investigator replaces the fuse in the lateral switch and closes lateral switch, the fuse will be blown again by the fault current. Now the investigator checks the readings of the fault indicators, which should read normal at Tx4 and Tx5 because no fault current goes through them, and the fault indicators on transformers Tx1 to Tx3 should read high fault current, if the fault indicators respond properly. The investigator disconnects (also called parks) the cable connected to the inlet of transformer Tx3, replaces the blown fuse in the lateral switch, then close lateral switch again. If the fuse holds, it confirms that the problem is either a transformer failure of transformer Tx3, or a fault cable between transformers Tx3 and Tx4. These two steps typically takes about 20 to 40 minutes. To determine whether the problem is a fault cable, or a transformer failure of transformer Tx3, the investigator disconnects cable connected to the outlet of transformer Tx3, reconnects the cable between transformers Tx2 and Tx3 to the inlet of transformer Tx3, and closes the lateral switch again. If the fuse holds, transformer Tx1, Tx2 and Tx3 are good. Therefore, the problem is a fault cable between transformers Tx3 and Tx4. This step typically takes about 15 to 30 minutes. At this time, the investigator can restore the power supply to transformers Tx4 and Tx5 prior to repairing the fault cable by disconnecting the cable connected to the inlet of transformer Tx4 and closing the normal open at transformer Tx5. The whole process of locating the fault cable described above can take about 2 to 4 hours, depending on the size of the loop system. Within this time the investigator has to drive among the transformers and to the lateral switch multiple times. Within this process, the lateral switch needs to be closed multiple times, each of them causes a fault current among a section of the loop system under the diagnosis test, which could cause further fault cables or transformer failures due to the high level fault current. It is not uncommon that more cables and transformers are damaged during the process of the existing diagnosis process. As noted, this process utilizes the fault indicators to assist the diagnosis, and assumes them respond reliably. Without the fault indicators, or in the case when their response is unreliable, the lateral switch needs to be closed even more. The process further lengthens, and potential damages to the cables and transformers due to the fault current further increase.
Therefore, there is a strong need for devices which can be utilized with the existing underground transformer loop system to simplify and speed up the process of diagnosis of fault cable or transformer failure.